The hydrological cycle provides a model for understanding the global plumbing system. Water spends time in the ocean, in the air, on the surface, and under the surface as groundwater. The hydrological cycle is a closed system because water is neither created nor destroyed on a large scale. Water exists as solid, liquid and gas phases that are interchangeable at temperatures found on earth. The hydrological cycle describes the movement of water as it passes through these phases. Water links the atmosphere, oceans and land through energy and matter exchanges as it evaporates, precipitates and flows.

The residence time of water in each part of the hydrological cycle determines its impact on climates. The short time spent by water in transit through the atmosphere results in short-term fluctuations in regional weather patterns. By contrast the long residence times, of 3,000 to 10,000 years in deep-ocean circulations, groundwater aquifers, and glacial ice act to moderate temperatures and climates. These slower parts of the water cycle work as a system memory, which both store and release heat, buffering climate change. There are four main components to the hydrological cycle: evaporation and transpiration, precipitation, surface water, and atmospheric water.